The present application is based on, and claims priority from, Italy Application No. TO99 A 00875, filed Oct. 11, 1999, the entire disclosure of which is hereby incorporated by reference herein.
The present invention relates to optical communications networks, and more particularly to a device for extracting, from a stream of optical signals comprising a plurality of carriers, one or more carriers, modulated by corresponding data signals, and inserting into the stream one or more other carriers, also modulated by corresponding data signals, normally different from the preceding ones.
These devices are commonly known in the art by the abbreviation OADM (Optical Add-Drop Multiplexer). The devices form a key element for the construction of nodes with wavelength-based routing functions in wavelength division multiplexing communications networks: in this case, they are used to extract from the multiplexed stream one or more channels containing the data intended for a user or users connected to the node, while allowing the remaining channels to pass unaltered, and to add to the multiplexed stream one or more channels which carry the data generated within the node. In practice, in order to optimize the use of the optical band, it is convenient to allocate the extracted and the added channel (or each pair consisting of an extracted and an added channel) to the same wavelength location.
One of the characteristics required in a device of this type for the new generation of optical networks is reconfigurability, in other words the possibility of selecting the channel or channels to be extracted and inserted by means of a remote command.
A system commonly used for making tunable extraction and insertion devices makes use of an element sensitive to the wavelength of the channel to be extracted and inserted, connected between a pair of circulators, one of which receives the multiplexed stream.
In one example, described in European Patent EP-B 0 638 837 in the name of the present applicant, the wavelength-sensitive element is a tunable optical bandpass filter of the Fabry-Perot resonant cavity type, formed in the fibre, which transmits the wavelength to which it is tuned and reflects all others. With this arrangement, one of the circulators forms the input/output ports for the multiplexed stream, while the other forms the carrier extraction and insertion ports. The tuning is carried out by varying the length of the cavity, e.g. by means of a voltage command.
In more recent versions of the same system, the wavelength-sensitive element is a Bragg grating in the optical waveguide, which reflects the resonance wavelength and transmits all others, so that the channel to be extracted is reflected from the grating towards the circulator which has received the multiplexed stream, and passes out of it, while the channels in transit proceed towards the second circulator, through which is inserted the new channel which, by reflection in the grating, will pass out of the second circulator together with the channels in transit. A structure of this type is described, for example, in the paper xe2x80x9cAdd-Drop Ottici per reti WDMxe2x80x9d, presented by G. Bendelli et al. at the xe2x80x9cFotonica 97xe2x80x9d conference, Rome, Italy, 20-23 May 1997, and published on pp. 18ff. of the conference proceedings. A grating can easily be tuned by thermal means or by application of mechanical stresses.
A limitation present in the aforementioned structures is that, during the tuning transient, the spectrum of the wavelength-sensitive element can pass through spectral regions to which channels used in the network are allocated. Since these known structures do not provide means for disabling the extraction function, the tuning transient can cause the suppression, for a certain time interval, of the intermediate channels between the xe2x80x9coldxe2x80x9d and xe2x80x9cnewxe2x80x9d tuning channel. In general, this degradation of the channels in transit, although temporary, is unacceptable, and limits the useful tuning range to the space between channels, enabling the spectrum of the wavelength-sensitive element to be moved only between adjacent channels. The limitation is becoming serious, since the development of optical components makes it possible to obtain both an increasing channel density and increasingly broad ranges of tunability.
The object of the invention is therefore to provide a device with improved properties of reconfigurability, and in particular a device which eliminates the limitation of the tuning range arising from the constraint of not interfering with the channels in transit during the transient.
The device according to the invention comprises:
input-output means which form an input port for the multiplexed stream to be subjected to the extraction and insertion operations, an output port for the multiplexed stream resulting from the said operations, and extraction and insertion ports for the carrier or carriers concerned, and
wavelength-sensitive means for the selection of the carrier or carriers within the stream, the said selection being tunable according to an external command, and is characterized in that the selection means are inserted into an element which is sensitive to the phase of the signals of the said stream and/or of the said carrier or the said carriers and is associated with phase control means which are capable of introducing into the said signals
a) for a given tuning condition of the selection means, a first phase shift value, such that the selected carrier or carriers to be extracted can pass from the input port to the extraction port and the selected carrier or carriers to be inserted can pass from the insertion port to the output port, and the other carriers can pass from the input port to the output port;
b) during a change of the tuning of the selection means, a second phase shift value such that all the carriers are caused to be transferred from the input port to the output port, thus disabling the extraction operation.
In a first embodiment of the invention, the wavelength-sensitive element is inserted in a Sagnac interferometer and the phase control means, during a change of tuning, are capable of establishing the same interference conditions for all the carriers in the stream.
In a second embodiment, the wavelength-sensitive element consists of a pair of tunable Bragg gratings and the phase control means interact with the said gratings in such a way that the latter, being tuned to a single wavelength not belonging to any carrier in the stream, act as a Fabry-Perot cavity transparent to the resonance wavelength.